Fluid level sensor

ABSTRACT

A fluid level sensor comprising a pivoted float which contains a solid-state inclinometer that produces sensitive electrical output signal responsive to angular displacement of the float from the direction of gravity. The float is capable of resolving very small changes in liquid level in a vessel, and thereby indicating or warning of small leaks from the vessel. A reference inclinometer may also be provided for producing a reference signal to compensate for fluid level signal errors caused by motion of the vessel.

DESCRIPTION

1. Technical Field

The invention relates generally to the sensing of changes of liquidlevel in vessels such as storage tanks, and more particularly to ahighly sensitive sensor which detects minute variations in angularinclination of a pivoting float with respect to the direction ofgravity.

2. Background Art

It is desirable that even minute leaks in liquid storage vessels bereliably detected, especially when the liquid is toxic, flammable orotherwise dangerous or highly valuable. A small change in stored liquidvolume corresponds to an extremely small change in the quiescent surfacelevel of the stored liquid. Yet a change of a few thousandths of an inchmay, in a large vessel, represent a substantial leakage volume.

Prior-art liquid level detectors have attempted various methods ofdetecting very small changes in the height of a vertically-travellingfloat. Crude pivoting floats, such as used in automobile gasoline tanks,automobile carburetor float chambers, toilet tanks and the like, haveheretofore been incapable of resolving the extremely small angulardisplacements of a pivoted float necessary for detecting a change inlevel of only a few thousandths of an inch reliably. The problem hasprimarily been the lack of a sensing and electrical transducingmechanism not only highly sensitive, but also immune to the variousinterfering factors, such as temperature, wave motion of the liquidsurface, etc. Optical, electromagnetic and other sensing mechanisms eachsuffer from interfering impingement of the very medium (light,magnetism, etc.) which they are designed to sense. Furthermore, suchprior-art level sensors are generally analog responsive to theparticular sensed medium, ambient strength or intensity of which mayvary to a greater extent than the minute variations sought to bedetected.

In a pivoting float type sensor, linearity of the angular sensingtransducer mechanism is important if an output is to be derived fromwhich the actual value of level change may be calculated or displayed.Such linearity is of less importance when only a preset thresholdangular displacement is to be monitored. One problem in a pivoting floatis that the center of buoyancy changes position with inclination in anodd-shaped float, whereas predictability and linearity require that theradius from pivot axis to center of buoyancy remain constant at allangular displacements.

Where a shield, or stilling basin is employed to isolate a sensitiveliquid level detector from an ambient surface wave motion of the liquid,it is desirable to be able to select and control the time constant, orrate of ingress and egress of the liquid between the sensitive innerportion of a stilling basin and the larger body of liquid.

Measurement errors of pivoting float type sensors also arise from motionof the vessel itself. Such a problem is evident, for example, inautomobile gasoline tank gauges and in carburetor floats notmechanically compensated, during hard cornering, hill-climbing and thelike. It is desirable in a sensitive leakage detector application to beable to distinguish between sensor output caused by actual change inliquid level, and that caused by change, even very minute change, inposition of the vessel itself.

While a level sensor can indicate a quiescent, desired liquid level at alevel somewhat above or below the middle of its measurement range, it isalso desirable to be able to make a gross initial height adjustment,over a range considerably wider than the narrow sensor range, toaccommodate relatively large differences in initial liquid level beforethe fine sensing of only thousandths of an inch change in levelcommences.

It is an object of the invention to provide a fluid level sensor capableof accurate, reproducible and reliable sensing of very minute liquidlevel changes.

Another object of the invention is to provide a fluid level sensor whichsenses a readily available medium, which does not rely upon intensity ofthe measured medium, and which is relatively impervious to variations inambient intensity of the measured medium.

A further object of the invention is to provide a fluid level sensorrelatively immune to wave motion of the liquid, and which provides forselected time constant of response to such wave motion.

Still another object of the invention is to provide a fluid level sensorhaving an electrical output conveniently convertible to alarm, readoutor feedback-control-loop usage.

Another object of the invention is to provide a fluid level sensorhaving linear or predictable relationship between electrical outputsignal and actual fluid level in a vessel.

A still further object of the invention is to provide a fluid levelsensor capable of sensing and compensating for motion of the liquidstorage vessel.

Yet another object of the invention is to provide a fluid level sensorhaving a gross initial level setting capability considerably wider thanthe usual sensing range of the operating sensor.

Another object of the invention is to provide a compact, self-containedpackage containing sensor and related electronics, easily insertable andreplaceable in existing storage tanks.

DISCLOSURE OF THE INVENTION

A solid-state inclinometer, which produces a substantial, accurate andreproducible electrical output signal, senses the very small differencebetween the inclinometer and the direction of gravity. Suchinclinometers, exemplified by the Sperry commercially available partnumbers PN 02338-03 and -02 are so new to the art that it is believed atthe time of filing hereof that any patents pending on the inclinometersthemselves have not yet issued. However, their commercial availabilitydemonstrate the feasibility of the solid-stategravitation-direction-detection inclinometer, which has primarily founduse thus far in aircraft and space systems.

The inclinometer is, in the present invention, affixed to the interiorof a float. The float defines a center of buoyancy in the liquid to besensed, and is pivoted at a defined distance from the center ofbuoyancy. The pivot is disposed at a defined spatial relationship to thevessel, such that the float inclines upward and downward as the changein fluid level raises or lowers the center of buoyancy with respect tothe pivot. A float support, in one embodiment, couples the pivot to thevessel. In another embodiment, the float is contained within a stillingbasin, and the pivot support is spatially coupled to the vessel byvirtue of affixation to the stilling basin, which is in turn affixed tothe vessel.

The stilling basin is a chamber surrounding the float, into which theliquid can flow through a restricted orifice, thereby slowing andreducing the momentary effects of wave motion in the larger body offluid outside the stilling basin. In one embodiment, replaceablecalibrated orifices are provided so that the degree of flow may beselected.

The electrical output of the inclinometer connects through umbilicalwiring to various signal processing electronics, which in variousembodiments calculates, triggers an alarm, drives a visual display, oroperates valves replenishing or draining liquid so as to maintain a moreconstant level.

In one embodiment, the entire electronics package is mechanicallycoupled to the float support and to the float, and releasably slideswithin a tubular float chamber to be set at a desired intial sensinglevel or height.

In another embodiment, a reference inclinometer is affixed, eitherdirectly or through the stilling basin, in fixed angular relationship tothe vessel itself. The signal output of the reference inclinometer iscompared to the output of the float-borne inclinometer, providingcompensation for error caused by motion of the vessel with respect tothe direction of gravity.

The problem of varying linearity, or varying sensitivity of inclinometeroutput to actual fluid level caused by shifting of locus of the buoyancycenter as the float's inclination changes, is addressed in an embodimenthaving a float geometry whose buoyancy center remains a constantdistance from the pivot point independent of inclination. One suchgeometry is a spherical float.

A vent is provided in the stilling basin to equalize pressure changesresulting from ingress or egress of liquid from the stilling basin.

One embodiment employs an inclinometer having an electrical outputsignal linearly proportional to inclination; in another embodiment theinclinometer output signal is digitally encoded for more convenientinterfacing with microprocessors and other digital electronics.

In general, the use of an inclinometer sensitive only to the relativedirection of gravity, rather than intensity of any sensed medium,produces a fluid level sensor of excellent sensitivity, reliability,predictability and reproducibility. The prior unavailability of thenewly available solid-state inclinometer technology has made possible apivoting-float type sensor which could not have thus previously beenconceived or developed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cutaway partial perspective view of the fluid level sensor.

FIG. 2 is a side sectional view of the fluid level sensor installed in astilling basin within a liquid storage vessel.

FIG. 3 is an enlarged partial sectional view of the view shown in FIG.2, more clearly showing pivoting action and the selectable orifice.

FIG. 4 is a partial top elevational view showing the float pivotinglysupported by the track-mounted sliding float support.

FIG. 5 illustrates the geometric principles involved in relating changedangular inclination to change in fluid level, showing relative positionsof the pivot and of the buoyancy center.

BEST MODE FOR CARRYING OUT THE INVENTION

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings. Specific language will be used to describe the same. Itwill, nevertheless, be understood that no limitation of the scope of theinvention is thereby intended, such alterations and furthermodifications of the illustrated device, and such further applicationsof the principles of the invention as illustrated therein beingcontemplated as would normally occur to one skilled in the art to whichthe invention relates.

The invention is generally referred to as number 10. Pivoting float 11contains within it solid-state inclinometer 12. An example of such asolid state inclinometer is the Sperry PN 02338-03, manufactured bySperry Aerospace and Marine Group, Sensing Systems, Phoenix, Ariz. Theexemplified inclinometer produces a full range linear output voltage of±3 volts DC from an inclination angle range with respect to thedirection of gravity of +60 degrees.

Float 11 has a float pivot support member 16 which is pivotingly coupledthrough pivot pin 15 to a corresponding single or clevis-shaped pivotsupport member 14, which in turn is affixed to float support 13. In theillustrated embodiment, float support 13 is slidingly engaged captive totrack 37, which is attached to the inner wall of stilling basin 28. Aswill be more clearly illustrated in subsequent figures, float support 13is, in operation, in a fixed spatial relationship to liquid vessel 24,such that a rising or falling of fluid level within the vessel causesfloat 11 to pivot upward or downward, thereby inclining or declining theinclinometer and producing an inclinometer output responsive to theliquid level change.

The words "liquid" and "fluid" are used interchangeably throughout theinstant application.

Umbilical wiring 18, which in practice should be soft and flexible so asnot to impede motion of float 11, provides electrical power toinclinometer 12, and transmits signal from inclinometer 12 throughwiring seal 17 to an electronics package comprising referenceinclinometer 19, electronics module 20, power supply 21 and visualindicator module 22 having visual display 23. Electronics module 20,depending upon application, may contain amplification, interfacecircuitry, digital circuitry, and in one embodiment, a microprocessorfor analysis or decision-making based upon signals received frominclinometer 11 and from reference inclinometer 19. For example, in theembodiment containing reference inclinometer 19, a microprocessor canperform subtractive or more complex calculations so as to compensate forfluid level measurement error caused by motion of the vessel itself.Reference inclinometer 19 senses only the motion or inclination of thevessel, independent of the fluid level, for purposes of providing such acalculation or correction procedure.

In FIG. 2, an embodiment is shown having a stilling basin 28 partiallyimmersed in liquid 35 within vessel 24. Stilling basin 28 iscylindrical, having threads 29 which screw into and mate with threadedvessel opening 27. Below the liquid level 43, liquid can communicate tothe interior of stilling basin 28 through calibrated orifice 34.Pressure relief is provided above the liquid level 43 by vent hole 32.Electronics package comprising elements 19,20,21,22 is shown as acylindrical module slideable within stilling basin 28, and in spacedrelationship to pivot 15 through the length of float support 13 slidingin track 37, which is affixed to an inside wall of stilling basin 37.Thus, it may be seen that raising or lowering the package, 19,20,21,22causes a corresponding vertical shift in height of pivot 15 relative toliquid level 43, thereby establishing an initial float level by manualadjustment. Once the desired position of said float-supporting structureis set, it is retained by tightening setscrew 31 within threaded hole30.

When fluid level decreases from its initial level 43 to level 44, float11 is downwardly inclined, with a corresponding inclinometer output from12. Inlet valve 25 and outlet valve 26 may be manually controlled, or inone embodiment, electrically controlled by the electronics package so asto form a closed feedback loop maintaining liquid 35 at a constant levelwithin vessel 24.

FIG. 3 more clearly shows calibrated orifice 34 threadedly engaged intothreaded hole 33. The size of orifice opening 40 is selected for adesired rate of flow between the interior of stilling basin 28 and thelarger body of fluid contained in vessel 24, surface of which may bedisturbed by waves and the like. The stilling basin 28 thereforeprovides a protected region free of such waves for float 11. In FIG. 3,float 11 is shown in a first float position 41 and a second floatposition (in phantom) 42.

In FIG. 4, float support 13 is seen to be slidingly captive within track37, which augments rigidity of the structure and reduces measurementerror which might be caused by vibration of the slender float support13.

FIG. 5 serves to illustrate the geometric principle of the invention. Insolid lines, a right triangle is formed by pivot 15, center of buoyancy45 of float 11, radius R and H₂, the height of the liquid level 43 abovepivot 15. As basic geometric principles dictate, the resultant angle Acorresponds to the angular difference between the direction of gravityand centerline CL shown in FIG. 3. In dotted lines, angle B is formedcorrespondingly when liquid level descends to H₁ above pivot 15. Thus,assuming that radius R remains constant, the angular difference betweenthe two corresponding inclinometer inclinations is calculated asfollows:

    (H.sub.2 -H.sub.1)=change in liquid height

H₂ =R (sin A)

H₁ =R (sin B)

therefore,

    (H.sub.2 -H.sub.1)=R (sin A-sin B)

As is known in the art, the locus of the center of buoyancy of anirregularly shaped float, such as that illustrated in FIGS. 1-4, shiftsin position as the irregularly shaped float changes its angle ofinclination, and thus the above calculation is disturbed by change inthe value of radius R. One shape of float which maintains constant locusof center of buoyancy would be a sphere (not illustrated, but within thecontemplation of the present invention).

Although not shown, a further embodiment contemplated in the presentinvention is the automatic or remote controlled adjustment of the sizeof orifice opening 40 by means of solenoids, shutters, irises and thelike.

What has been disclosed is a sensitive fluid level sensor whichelectrically senses the deviation from direction of gravity of a pivotedfloat at a liquid-gas interface within a storage vessel.

Those skilled in the art will conceive of other embodiments of theinvention which may be drawn from the disclosure herein. To the extentthat such other embodiments are so drawn, it is intended that they shallfall within the ambit of protection provided by the claims herein.

Having described our invention in such a clear and concise manner in theforegoing description and drawings that those skilled in the art mayreadily understand and practice the invention, that which we claimis:
 1. In a fluid level sensor for sensing a range of changes in thelevel of fluid in a vessel; said sensor comprising an inclinometerhoused within a stilling basin coupled to a fluid containing vessel andhaving means for ingress and egress of said fluid to and from saidstilling basin, said inclinometer outputting a signal indicative of thefluid level within said stilling basin; the improvement comprising:anelectronics package slidably coupled to and within said stilling basin;a slide track affixed to said stilling basin; aninclinometer-bearing-float in fluid contact with the fluid within saidstilling basin, said inclinometer being electrically coupled to saidelectronics package; and a float support shaft having a first endthereof coupled to said electronics package, a second end thereofpivotally coupled to said float, and the length thereof rigidly,slidably coupled to said slide track,whereby the slidable adjustment ofsaid electronics package within said stilling basin produces acorresponding adjustment in the fluid contact of saidinclinometer-bearing-float with the fluid within said stilling basin tothereby initially set the desired level of fluid in said vessel.
 2. Theimprovement of claim 1 further comprising:a reference inclinometercoupled to said fluid containing vessel for providing a reference signalindicative of the inclination of said vessel; and means within saidelectronics package for comparing the reference signal from saidreference inclinometer and the signal from said inclinometer within saidfloat to compensate for fluid level signal errors caused by motion ofsaid fluid containing vessel.
 3. The improvement of claim 1 furthercomprising:electronically controlled means of ingress and egress offluid to and from said fluid containing vessel coupled to said vessel;and means within said electronics package coupled to said electronicallycontrolled ingress and egress means for electronically controlling thefluid level within said vessel responsive to the signal indicative ofthe fluid level within said stilling basin.
 4. The improvement of claim1 further comprising:a reference inclinometer coupled to said fluidcontaining vessel for providing a reference signal indicative of theinclination of said vessel; means within said electronics package forcomparing the reference signal from said reference inclinometer and thesignal from said inclinometer within said float to compensate for fluidlevel signal errors caused by motion of said fluid containing vessel;electronically controlled means of ingress and egress of fluid to andfrom said fluid containing vessel coupled to said vessel; and meanswithin said electronics package coupled to said electronicallycontrolled ingress and egress means for electronically controlling thefluid level within said vessel responsive to the signal indicative ofthe fluid level within said stilling basin.
 5. The improvement of claim1 further comprising output signal means coupled to said electronicspackage for outputting a visible indication of the fluid levelresponsive to the signal indicative of the fluid level within saidstilling basin.
 6. The improvement of claim 4 further comprising outputsignal means coupled to said electronics package for outputting asensible signal responsive to the signal indicative of the fluid levelwithin said stilling basin.